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Mode S all-call interrogations

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Mode S interrogations.

 1 Mode A or C interrogation
_ A mode A or C interrogation consists of two pulses labelled P1 and P3. A control pulse P2 is transmitted following the first interrogation pulse. P1 and P3 are radiated by a narrow rotating beam antenna. P2, also called the Side Lobe Suppression (SLS) pulse, is radiated by an omnidirectional antenna with an amplitude of 9 dB below P1 and P2 within the main lobe. P1 amplitude is greater than or equal to P2 amplitude from the greatest side lobe transmission of the antenna radiating P1.
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 2_ Mode S all-call interrogation
 (Ref. Fig. 007)
 A Mode S all-call interrogation consists of three transmitted
 pulses labelled P3 and P4. A control pulse P2 (Side Lobe
 Suppression Pulse) is transmitted following P1. P1 amplitude
 is greater than or equal to P2 amplitude from the greatest
 side lobe transmission of the antenna radiating P1.
 P2 amplitude is at a level lower than 9 dB below P1 amplitude
 within the main lobe.
 P4 duration is 0.8 or 1.6 microseconds depending on the
 interrogation mode.
 If P4 is equal to 0.8 microseconds, this means that the
 interrogation only concerns conventional ATC mode A or C
 transponders, and consequently the Mode S transponder should
 not reply.
 If P4 is equal to 1.6 microseconds, this means that the mode S
 transponder should reply with mode S information.
 3_ Mode S interrogation
 (Ref. Fig. 008)
 A Mode S interrogation consists of a two-bit preamble followed
 by a data block.
 The preamble consists of two transmitted pulses labelled P1
 and P2. The characteristics of P1 and P2 are the same as for
 the mode S all call interrogation, except that P2 amplitude is
 equal to P1.
 The data block consists of a single pulse P6 with a
 Differential Phase Shift Keying (DPSK) modulation.
 . Pulse duration: short 15.5 microseconds / long 29.5
 microseconds
 . Data block: 56 or 112 bits.

 (b) Encoded reply
 1 Mode A or C reply
_ A Mode A or C reply consists of two framing pulses F1 and F2, separated by a 20.3 microsecond time interval. The encoding of the reply is done by means of the presence (1) or absence (0) of 13 reply pulses (12 pulses plus X pulse).
 2 Mode S all-call reply
_ A Mode S all-call reply consists of a four-bit preamble followed by a data block. The data block contains 56 position modulated pulses.
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 3_ Mode S reply
 A Mode S reply occurs after a mode S interrogation or a mode S
 all-call interrogation (P4 duration 1.6 microseconds).
 A mode S reply consists of a four-bit preamble (duration 8
 microseconds) followed by a data block. The data block
 contains 56 or 112 position modulated pulses.
 The replies are PPM coded.

 (c) Mode S message content The minimum data link transponder supports all surveillance functions, in addition to bidirectional air-to-air data exchange, ground-to-air data uplink (Comm A), air-to-ground data downlink (Comm B), and multisite message protocol. In addition, the transponder is capable of receiving Extended Length Messages (ELMs) from the ground. ELMS are received in the Comm C format. ELM transmittals to the ground use the Comm D format. (Note: The ATC Collins is capable of comm A(B) C data-link operation). All discrete Mode S interrogations and replies (except the all-call reply) contain the 24-bit discrete address of the Mode S transponder upon which 24 error detection parity check bits are overlaid. In the all-call reply, the 24 parity check bits are overlaid on the Mode S interrogation address and the transponders7 discrete address is included in the text of the reply. The main function of Mode S is surveillance. To accomplish this function, the Mode S transponder uses the 56-bit transmissions ( each direction). In the 56-bit transmissions, the aircraft repor its altitude or ATCRBS 4096 code, and the flight status (airborn on-ground, alert, Special Position Identification (SPI), etc. The discrete addressing and digital encoding of Mode S transmiss permit their use as a digital data link. The interrogation and r formats of the Mode S system contain sufficient coding space to permit the transmission of data. These data transmissions may be for air traffic control purposes, air-to-air data interchange fo collision avoidance, or to provide flight advisory services such weather reports, or Automated Terminal Information System (ATIS) Most Mode S data link transmissions will be handled as one 56-bi message included as part of a long 112-bit interrogation or repl These transmission include the message in addition to the surveillance data. Longer messages are transmitted using the Extended Length Message (ELM) capability. The ELM is capable of transmitting up to sixteen 80-bit message segments, either ground-to-air or air-to-ground. The ELM can be acknowledged with a single reply or interrogation. ELMs uplinked need not be replied to individually, but can be acknowledged in a reply containing a summary of the received interrogations. ELMs do not contain surveillance data.
　
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